The Nautical Sextant

Recently, when I mentioned that I restored sextants , I was asked if I resilvered their mirrors. I do, but only for antique sextants, when substituting  modern mirrors might conceivably affect the value. For others, as often as not, I substitute a modern mirror and carefully wrap the old mirrors to keep with the instrument, in case they later acquire significance.

In a recent post (How flat are sextant mirrors?), I pointed out that modern mirrors are made of float glass which, over small areas, is flat to within half the wavelength of green light and which compares well with the glass from a variety of sextants from as far back as the 1850s. Float glass is made by floating molten glass on a bed of molten tin.

There is a knack in cutting small pieces of glass which I have not yet acquired, but I have a friendly local glazier who will cut pieces of 3.8 mm thick mirror glass to size for me. He charges me about 50 US cents a piece. Then, in less time than it takes to write this post, I grind and bevel the edges. The longest part of the process is waiting for the paint to dry and harden afterwards. Here’s how it’s done.

It is now possible to buy a set of diamond-faced “sharpening stones” or laps for well under $20. You will need a medium (240 grit) and fine (360 grit) one, a shallow dish, a drop of washing-up liquid and some water. Fill the dish until the water just covers the medium lap and add a drop of washing up liquid.

Hold the mirror as shown in the next photo, at right angles to the lap and rub the mirror up and down the length of the lap to grind the edge. You don’t need to apply much pressure, just enough to keep the whole edge in contact with the lap. If you press too hard, you risk chipping the edge.

After about a dozen strokes, check to see that the edge is evenly ground and if it is, use a few strokes on the fine lap to finish off.  The next photo shows what it should look like. The unground edge, as cut, is on the right and the ground edge, seen frosted, is on the left. By the way, the black spots on my thumb are caused by splashes of silver nitrate solution, used in re-silvering glass.

When you have completed all the edges, tilt the mirror so that its face is at 45 degrees to the lap and grind a bevel on the edge using the medium and fine lap. It needs a slightly different grip to stop the mirror rotating, shown in the next photograph. Make a smaller bevel on the rear surface, using only the fine lap, so as not to tear the paint and silvering. The purpose of grinding and bevelling, apart from neatness of appearance,  is to provide a seat for the sealing paint, to avoid sharp corners which will thin the paint film and to remove tiny chips and cracks from which strains and bigger cracks may start.

Next round the corners. This needs yet another grip, shown in the next photograph. The mirror is held between the thumb, forefinger and middle finger and rotated in an arc by flexing and extending them. Move your grip to extend the arc until there is a smooth blend from the edges into the arc. In the example shown, all that I wanted was to remove a sharp corner, but some mirrors need relatively large radii on the corners if they are  to fit the mirror mount. This is quite easily done with this method; it just takes longer.

Here’s the end result, ready to paint:

The paint has the very important function of preventing access of sea water to the silvering, so it is important that it extends around the edge of the mirror, well beyond the extent of the silvering. It seems pointless to go to this trouble and then allow the points of the adjusting screws to puncture the paint film. I interpose a thin sheet of brass. One manufacturer (Hughes and Son) sometimes cemented little discs of brass to the areas where the screws bore and another (Heath and Co) in their top-of-the-range sextants cemented a whole sheet of glass over the silvering, so that it was sandwiched between two sheets. Most either used nothing or an inadequate piece of card. When painting, cover the front of the mirror with masking tape and apply paint copiously, with special attention to the edges. All paints are porous to some extent, but the one shown in the next photograph seems better than most and, when fully cured after a few days, is very tough.

You’ll be surprised how easy it is after very little  practice.

Split horizon mirrors are done exactly the same way after removing the paint and reflective coating, using the original to judge how much to remove. Start by scoring with a safety razor blade, using a small square to guide the blade:

Then use the blade as a mini-chisel to scrape off the paint in strips. Keep all your fingers behind the blade, as blood will rust it and you won’t be able to use it again:

You will find that much of the reflective coating comes off with the paint. Complete the job by standing the mirror upright in a beaker and pouring in concentrated hydrochloric acid up to the paint line. The acid is sold for etching concrete as “Spirits of salt”. Wear eye protection and plastic or rubber gloves to handle this nasty stuff, and cover the beaker, as it fumes in humid conditions and will rapidly rust steel tools in the vicinity.

After ten minutes, pull out the mirror and rinse in clean water. Rubbing with a thumb will rub off most of the film. Any that remains can have another ten minutes in the acid.

When painting the backs of horizon mirrors, ensure that the paint overlaps the edge of the silvering a little, so that sea water cannot penetrate to it via the edge. Masking the plain portion of the glass with plastic insulating tape is the best way of ensuring a crisp edge to the paint.